Recent advances in low power circuits have enabled mm-scale wireless systems for wireless sensor networks and implantable devices, among other applications. Energy harvesting is an attractive way to power such systems due to the limited energy capacity of batteries at these form factors. However, the same size limitation restricts the amount of harvested power, which can be as low as tens of nW for mm-scale photovoltaic cells in indoor conditions. Efficient DC-DC up-conversion at such low power levels (for battery charging) is extremely challenging and has not yet been demonstrated.
Boost DC-DC converters are widely used to harvest energy from DC sources and yield high conversion efficiency. However, they require a large off-chip inductor at low harvested power levels, increasing system size. Alternatively, switched-capacitor (SC) DC-DC converters can be fully integrated on-chip and are favored for form-factor constrained applications. At low power levels, SC converter efficiency is constrained by the overheads of clock generation and level-conversion to drive the switches. As a result, efficient SC converter operation has been limited to the μW range.
This disclosure presents a fully integrated switched-capacitor energy harvester that consists of cascaded self-oscillating voltage doublers. In each voltage doubler, an oscillator is completely internalized and clocking power overhead is reduced. The reduced power overhead of both clock generation and level shifting enables the harvester to operate with very weak power sources, as low as a few nWs. By completely integrating the clock generation in the SC, the overhead scales with the current load resulting in a very wide load range of ˜1000×. By adjusting the number of cascaded voltage doublers as well as with a new method of modulating the low voltage applied to each doubler stage, the overall conversion ratio can be configured between 9× and 23×.
This section provides background information related to the present disclosure which is not necessarily prior art.